Multichip on-board led illumination device

ABSTRACT

An LED-based illumination device can use an array of four LEDs to produce high intensity light over a broad color spectrum and a broad range of color temperature. A high quality white light can be produced by using two green LEDs with a single red and a single blue LED.

PRIORITY CLAIM PROVISIONAL

This application is a continuation of U.S. application Ser. No.11/265,914, filed Nov. 3, 2005, which in turn claims the benefit of U.S.Provisional Application No. 60/728,861, filed Oct. 21, 2005, both ofwhich applications are incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to illumination systems and devicescapable of emitting light at a high power over a large region of thevisible spectrum.

BACKGROUND

An increasing number of applications require illumination systems thatare small in size but high in intensity. Light sources such as LEDstypically have been used as indicator lights, but have not been used forprojection or illumination systems because LEDs typically lack therequired intensity. LEDs are desirable sources for many applications,however, due to their small size, low cost, and ease of use. Anotherproblem with LEDs is that they typically are not able to produce a highquality color spectrum. Some existing systems utilize a single blue LEDwith a phosphorus conversion layer to produce white light. However, theoutput spectrum of this device is fixed and cannot be varied during use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illumination device in accordance with oneembodiment of the present invention.

FIG. 2( a) is a perspective view and FIG. 2( b) is an explodedperspective view of the device of FIG. 1.

FIG. 3( a) is a side view corresponding to the cross-section in the topview of FIG. 3( b), and FIG. 3( c) is a side perspective view of thedevice of FIG. 1.

FIG. 4 is a plot of typical spectral distributions that can be obtainedwith the device of FIG. 1.

FIG. 5 is a plot showing the color coordinates in color space of theillumination device of FIG. 1.

FIG. 6 is a plot showing a typical beam pattern for the device of FIG.1.

FIG. 7 is a plot showing the relative intensity by angular displacementof the device of FIG. 1.

FIG. 8 is a top view and schematic diagram showing the LEDs of thedevice of FIG. 1.

FIG. 9 is a top view of the device of FIG. 1 on blister tape packaging.

FIG. 10 is a perspective view of the blister tape of FIG. 9.

FIG. 11 is a plot showing the spectrum of a white light LED of the priorart.

FIG. 12 is a plot showing the spectrum of the device of FIG. 1.

DETAILED DESCRIPTION

Systems and methods in accordance with embodiments of the presentinvention can overcome various deficiencies in existing LED andillumination systems. A device in accordance with one embodiment is amulti-use, compact LED device operable for use in applications such asspecialty lighting applications, vision systems, general illumination,architectural lighting, transportation lighting, mood lighting, medicallighting, backlighting, and display/signage applications. Such a devicecan be a high-power light source, such as may run at up to 4.5 W in oneembodiment, utilizing multi chip-on-board (COB) technology. A device inaccordance with one embodiment utilizes four LEDs, one red LED, one blueLED, and two green LEDs, in order to produce a high quality white light.Such a device also can operate over a high and wide color temperaturerange, and can generate a high intensity light with excellent and variedcolor composition. Such a device also can be highly efficient.

An illumination device in accordance with one embodiment is shown in thetop view 100, 320, perspective view 200, 250, and side view 300, 350diagrams of FIGS. 1-3, respectively. Numbers will be carried overbetween figures where appropriate for simplicity. The illuminationdevice consists of a substrate 102 formed of an appropriate material,such as a copper PCB having a standard PCB thickness. In one embodiment,the copper is about 1 mm thick. A substrate such as copper can guaranteegood thermal conditions for such a device. The substrate can have anisolation layer 104 deposited or otherwise formed thereon. The isolationlayer can be made of any appropriate thermally conductive material, suchas a layer of epoxy material mixed with ceramic powder for electricalisolation. The isolation layer can be thin, such as on the order ofabout 60-70 μm thick.

Upon the isolation layer 104 can be placed a circuit layout 106, such asmay be formed from a material such as copper. The circuit layout canhave any appropriate thickness, such as a thickness of about 35 μm, andcan have a thin layer of gold thereon, such as a layer of about 0.4-0.6μm thick. The layout can have a central pad region 108, as well as ananode pad 110 and a cathode pad 112 for each color LED (identified herefor blue) to be used in the device. Upon the circuit layout can beplaced appropriate wire bonding material (not shown), such asnickel/gold wire bonding as known in the art. A solder mask or colorprint (not shown) also can be placed on top of the copper circuit asdesired.

A square array of four LED chips 114, 116, 118, 120 is placed on thecentral pad region 108 in this embodiment, although other arrangementsand combinations are possible to obtain a desired intensity and/or colorcombination. The LEDs can be attached to the pad through any appropriatemechanism, such as silver epoxy glue of about 10 μm in thickness. TheLEDs can be any appropriate high intensity LEDs, such as LEDs having athickness of about 120 μm and an active area of less than 1 mm² thatoperate at currents up to about 500 mA per die. The design for heatdissipation allows the LEDs to be packed tightly together in order toimprove color mixing. The die can be mounted on a very small area of thecircuit board, such as an area of less than 9 mm² (having dimensions of2.1 mm×2.1 mm, for example), while the underlying layers substantiallydissipate heat generated by the LEDs without damaging the device. Sincethe majority of the heat is dissipated, the wavelength shift of the LEDsdue to temperature effects can be reduced. This helps to control thecolor output of the device over time.

One of the LEDs, in this embodiment the red LED 114, can have anelectrical connection between the back side of the LED and the centralpad region of the circuit layout. The other LED chips, here the blue andtwo green LEDs, can be isolated from the circuit layout so that only theback side of the red LED is in contact with the pad. The red LED 114then can have a single wire bond on the front surface, while the blue118 and green 116, 120 LEDs each have two wire bonds on the frontsurface. Each wire bond can go to the anode or cathode for therespective LED in order to allow for activation and control of the LEDsthrough connection with the anodes and cathodes as known in the art. Thebonds can use any appropriate material to connect to the appropriateanode/cathode, such as 25 μm diameter gold wire. Allowing only one ofthe LEDs to be in electrical contact with the central pad region 108 ofthe circuit layout 106 allows all four chips to be glued to the samepad, simplifying the design and manufacture of the device and allowingthe LED chips to be placed in sufficient proximity to one another toobtain good color blending. In this embodiment, the LED chips are placedonly 0.1 mm apart in order to minimize the footprint of the LEDS andmaximize color blending. Bonding the backs of all the LEDs to individualbond pads can limit design options and flexibility.

After the wire bonds are formed to electrically connect the LED chips tothe appropriate anode and cathode pads, a retaining ring 122 can beplaced around the LED array. The retaining ring can be attached to theisolation layer and circuit layout using any appropriate mechanism, suchas epoxy glue. Once in place, an encapsulant material 126 such assilicon can be poured, flowed, or otherwise placed into the retainingring 122 in order to cover the LED chips and wire bonds. As known in theart, a silicon encapsulant can protect the device electronics, whileserving to out couple the light from the LEDs. The retaining ring can beshaped to provide sufficient area and strength for the encapsulant tocompletely cover the LEDs and bonds in a relatively attractive manner.The structure for retaining the encapsulant also can have other shapes,such as square, octagonal, or any other appropriate shape.

The PCB substrate 102 in this embodiment is shown to have a generallyhexagonal structure, but any appropriate polygonal or circular shape canbe used as may be required by the end application. Using a hexagonalshape allows the devices to be packed in the tightest possibleconfiguration for three devices, for example. Other shapes could workbetter for different numbers of devices. The substrate 102 also is seenin this embodiment to include circular recesses 124. These recesses canbe used to pass cables and wiring through the device array when thedevices are packed, as well as allowing access for screws or othermounting devices. As can be seen for this embodiment, packing threedevices together can create a full circular opening therebetween forpassing wiring, etc.

Such a device, here using two green LEDs with a red and a blue LED, iscapable of producing a high quality white light that cannot be obtainedwith a standard RGB device. FIG. 4 shows a plot 400 of typical spectraldistributions that can be obtained with LEDs selected for a device suchas described with respect to FIGS. 1-3. These LEDs can be selected basedon factors such as wavelength and intensity. Using this distribution,along with two green LEDs, such a device can cover almost 85% of thevisible color space 500, such as is shown in FIG. 5. The outerelliptical shape 502 represents all visible wavelengths, while the innertriangular shape 504 represents the colors that can be produced using adevice such as described with respect to FIG. 1. The curved line 506 inthe middle is referred to as a “white line,” as the line represents allthe combinations of the LEDs at all the various color temperatures thatcombine to produce white light. A good “sunlight” white can be obtainedwith a color temperature in a range around 2800-3500K. Table 1 shows theefficiencies and intensities for various color temperatures andcurrents.

TABLE 1 Color Temperature Red Green Blue K mA mA mA 2800 100 61 13 3500100 74 21 4000 100 82 26 5000 100 88 34 6500 100 96 36 7000 100 88 52Such a device can allow similar currents to be applied to each of thesestate-of-the-art LED chips. The device also can operate at a relativelyhigh power to produce high intensity light. The placement and use of theanode and cathode pads allow a user to easily apply and vary a desirableamount of current to each LED to obtain the desired illumination.

Such a device also can tend to generate light in a fairlynon-concentrated manner. A device in accordance with one embodiment is aLambertian emitter with a 120° aperture. For instance, FIG. 6 shows atypical beam pattern 600 and FIG. 7 shows relative intensity 700 byangular displacement for a device such as is described with respect toFIG. 1. For various applications, such as projection, where it isdesired to have a more focused beam of light produced, a number ofoptical elements can be used to adjust the output of the illuminationdevice. Any of a number of optical elements known or used in the art canbe positioned to collimate, focus, direct, filter, or otherwise modifythe output of the device.

FIG. 8 shows another top view 800 of the device of FIG. 1 with exemplarydimensions. FIG. 8 also shows a circuit diagram 802 for the LEDs showingthat the green LEDs are connected in series.

In order to easily store a number of these devices and deliver them to acustomer, the devices can be attached to a blister pack or blister tapeas known in the art. FIG. 9 shows the attachment 900 of such a device toa piece of blister tape. FIG. 10 shows the blister tape stored in rolls1000 for easy storage and delivery. Electrical contacts on the backsideof the device can allow for a variety of surface mounting techniques.

As discussed earlier, different systems have different strengths acrossthe color spectrum. For example, a blue LED with a phosphor emittingyellow/green produces a spectrum 1100 such as is shown in FIG. 11. Ascan be seen, such a device produces strong yellow light and a mediumamount of blue light, but does not produce much light in the red regionof the spectrum. A device in accordance with one embodiment of thepresent invention can produce a spectrum such as is shown in FIG. 12,which produces similar intensity in the red, green, and blue regions ofthe spectrum. This provides for good color mixing, and the production ofhigh quality white light.

Tables 2 and 3 present specifications that have been obtained forillumination devices in accordance with various embodiments.

TABLE 2 Technical Data Optical and electrical characteristics Ambienttemperature = 25° C. Parameter Symbol RED GREEN BLUE Unit Luminous @350mA typ. Φ_(v) 16 26 2.7 lm flux** Luminous @350 mA typ. Φ_(v) 27 41 4 lmflux*** Dominant @350 mA λ_(dom) (0) 620-630 (0) 515-530 (0) 455-465 nmwavelength* (1) 620-630 (1) 520-525 (1) 460-465 nm (2) 515-520 (2)455-460 nm (3) 525-530 nm Spectral @350 mA typ. Δλ 30 45 30 nm bandwidthForward @350 mA typ. V_(F) 2.5 7.0 3.6 V voltage max. V_(F) 3.0 8.0 4.0V Optical @350 mA typ. η_(opt) 18 11 2 lm/W efficiency** Optical  @50 mAtyp. η_(opt) 33 23 5 lm/W efficiency** Optical @350 mA typ. η_(opt) 3117 3 lm/W efficiency*** Viewing typ. 2_(Ψ) 120 120 120 degree angle at50% Radiating typ. A_(rad) 1.0 2 × 1.0 1.0 mm² surface Thermal assembly:10 K/W resistance *binning: (0) = standard, (1) = main binning Adequateheat sink is required. Derating must be observed to maintain junctiontemperature below maximum. **Please note: These values are measured atrealistic working conditions for thermal equilibrium with a small heatsink. Attention: Some suppliers present only optimized laboratoryconditions. ***Values for board-temperature of 25° C.

TABLE 3 Maximum Ratings @ 25° C. Symbol Values Unit Operating T_(op) −40to 100 ° C. temperature range Storage T_(st) −40 to 100 ° C. temperatureJunction T_(J) 120 ° C. temperature Forward current I_(p) 350 mA percolor Surge current per T_(op) 700 mA color Forward voltage RED BLUEV_(R) 5 V GREEN V_(R) 10 V Power RED P_(tot) 1.05 W consumption GREENP_(tot) 3.2 W BLUE P_(tot) 1.8 W

It should be recognized that a number of variations of theabove-identified embodiments will be obvious to one of ordinary skill inthe art in view of the foregoing description. Accordingly, the inventionis not to be limited by those specific embodiments and methods of thepresent invention shown and described herein. Rather, the scope of theinvention is to be defined by the following claims and theirequivalents.

1. An illumination device, comprising: a single red light emitting diode (LED); a single blue LED; one pair of green LEDs; and a substrate supporting only the red, blue, and pair of green LEDs in a square array; a circuit layout operable to receive current from an external source and pass that current to an appropriate one of the red, blue, and pair of green LEDs for generating light.
 2. An illumination device as recited in claim 1, wherein spacing between the LEDs is on the order of 0.1 mm.
 3. An illumination device as recited in claim 1, wherein the four LEDs are packed within an area less than 9 mm².
 4. An illumination device as recited in claim 1, wherein said substrate has a hexagonal configuration.
 5. An illumination device as recited in claim 1, wherein the illumination pattern created by the LEDs is Lambertian.
 6. An illumination device as recited in claim 1, wherein said green LEDs are connected in series and said external source provides selectable drive currents to the red LED, the blue LED chip, and the series-connected LEDs.
 7. An illumination device as recited in claim 1, wherein drive current supplied by the external source to the individual LEDs is variable to produce a selected color output.
 8. An illumination device as recited in claim 1, wherein said LEDs are sealed with an encapsulant.
 9. An illumination device, comprising: a substrate; a circuit layout positioned on the substrate, the circuit layout including a plurality of anode and cathode pads; a square array of four closely packed LEDs supported by the substrate and electrically connected to the circuit layout in order to allow activation through the application of current to an appropriate pair of the plurality of anode and cathode pads; and an encapsulant positioned over the square array of four LEDs to prevent external contact with the square array while out coupling light from the square array.
 10. An illumination device as recited in claim 9, wherein the four LED's are packed within an area less than 9 mm².
 11. An illumination device as recited in claim 9, wherein spacing between the LEDs is on the order of 0.1 mm.
 12. An illumination device as recited in claim 9, wherein said substrate has a hexagonal configuration.
 13. An illumination device as recited in claim 9, wherein the illumination pattern created by the LEDs is Lambertian.
 14. An illumination device, comprising: a substrate; a circuit layout positioned on the substrate, the circuit layout including a plurality of anode and cathode pads; a square array of four closely packed diodes supported by the substrate and electrically connected to the circuit layout via an associated pair of the plurality of anode and cathode pads, wherein at least three of the diodes are light emitting diodes; and an encapsulant positioned over the square array of four diodes to prevent external contact with the square array while out coupling light from the square array.
 15. An illumination device as recited in claim 14, wherein the four diodes are packed within an area less than 9 mm².
 16. An illumination device as recited in claim 14, wherein spacing between the diodes is on the order of 0.1 mm.
 17. An illumination device as recited in claim 14, wherein said substrate has a hexagonal configuration. 